Iron is found in an oxidized state and is mined from the ground as an iron ore.
Energy is used to convert this iron oxide into steel. Ore is loaded into a blast furnace where
heat energy removes the oxygen producing elemental iron.
Finally, it is rolled in a pipe mill or rod mill to form pipe, rebar or structural shapes.
Corrosion occurs when iron contacts an electrolyte such as earth or concrete. In that
corrosion process, energy is lost and the iron combines with oxygen to form iron oxide. This
corrosion product, rust,
is identical to the iron oxide mined
from the ground.
Iron is considered thermodynamically
unstable; given the right environment,
iron will readily corrode to return to its
lower energy state, as rust.
Corrosion can be defined as the deterioration of metal due to
interaction with the environment.
Corrosion is a natural phenomenon that is expected to occur.
Metals as high energy materials exist because heat energy
was added to natural iron ores during the smelting process.
Environmental contact constantly attacks these high energy
materials and breaks them back down to the natural elements
from which they were derived.
The anode and the cathode can be on different metals or on
the same metal as shown:
For many metals, the rate of corrosion increases appreciably
below a pH of about 4.
Between 4 and 8 corrosion rate is fairly independent of pH.
Above 8, the environment becomes passive and corrosion
rates tend to decrease.
Corrosion cells may form because of differences in the
electrolyte. For example, when a single metal structure spans
an electrolyte made up of different types of soils, different
chemical substances, different concentrations of the same
substance, or temperature variations, the structure may
experience voltage differences.
Potential Difference Between Anode and Cathode (Galvanic
Series)
Circuit resistance – Resistivity of the Electrolyte
Chemical Activity
Example connecting zinc with carbon will produce a corrosion cell with a potential of
about 1.4 volts.
METAL
VOLTS (CSE)
Commercially Pure Magnesium -1.75
Magnesium Alloy
-1.60
Zinc
-1.10
Aluminum Alloy
-1.05
Commercially Pure Aluminum
-0.80
Mild Steel (clean & shiny) -0.50 to -0.80
Mild Steel (rusted)
-0.20 to -0.50
Cast Iron (not graphitized)
-0.50
Lead
-0.50
Mild Steel in Concrete
-0.20
Copper, Brass, Bronze
-0.20
High Silicon Cast Iron
-0.20
Carbon, Graphite, Coke
+0.30
Circuit resistance includes the following:
Resistance of the anode
Resistance of the cathode
Resistance of the electrolyte
Resistance of the metallic path
Increasing the resistance will reduce the corrosion rate.
Soils – High resistivity soils reduce the corrosion rate,
while low resistivity soils increase the corrosion rate.
CLASSIFICATION
ELECTROLYTE
RESISTIVITY
(ohm-cm)
ANTICIPATED
CORROSIVITY
Low Resistance
50 to 2,000
Severe
Medium
2,000 to 10,000
Moderate
High
10,000 to 30,000
Mild
Very High
Above 30,000
Increasingly Less
Passive (less corrosive) Environment
High pH (neutral or basic)
Low Moisture Content
Lack of Salts
High Resistivity
Low Temperature
Homogenous Environment
Uniform or near uniform (slow)
Localized (moderate)
Pitting (can be very rapid)
Uniform or near uniform - Corrosion attacks all areas of the
metal at the same or similar rate typically by atmospheric
contact. Weathering steel alloy.
Localized - Some areas of metal corrode at different rates due
to heterogeneities in the metal or environment. This type of
attack can approach pitting.
Pitting - Very highly localized attack resulting in small pits that
may quickly penetrate to perforation.
Reference electrodes, or half-cells, are important devices that
permit measuring the potential of a metal exposed to an
electrolyte. An example is a structure-to-soil potential
measurement.
Structure-to-soil potentials are measured in reference to an
electrode.
Saturated CSE Reference
Over the center line of the buried structure.
Piping
Underground or submerged steel, cast iron, aluminum, and
pre-stressed concrete cylinder pipelines.
Buried Tanks
Underground Storage Tanks (UST) and piping
Aboveground Storage Tanks
Exterior tank bottoms (both primary and secondary) of
Aboveground Storage Tanks (AST).
Piling
Foundation piles
Concrete
Concrete bridge deck reinforcement and substructures
Corrosion can occur at the steel/concrete interface, resulting
in spalling.
Reinforced concrete is susceptible to chlorides when used in
marine environments.
Sulfate exposure physically damages concrete.
Galvanic (or sacrificial) cathodic protection makes practical use of dissimilar
metal corrosion. It requires a substantial potential difference, or driving voltage,
between a galvanic anode and the structure to be protected.
The galvanic anode is connected to the structure it is protecting, either directly or
through a test station so it can be monitored.
There are several metals commonly used as galvanic anodes:
• Aluminum
• Magnesium
• Zinc.
Typically most effective with electrically isolated and wellcoated structures.
No external power source required but limited driving potential
(driving potential based upon the galvanic series).
Limited output makes it ineffective when trying to protect large
surfaces.
Requires a low resistivity electrolyte to function well.
IM PR ESSED CU R R EN T SY STEM
An o d e Gro u n d b e d
Po s i ti v e Ca b l e
R e cti fi e r
AC Po w e r Su p p l y
Pi p e l i n e
N e g a ti v e Ca b l e
Graphite Anodes
Conductive Polymer
High-Silicon Chromium-Bearing Cast Iron
Lead-silver
Mixed-metal oxide (MMO)
Platinum
Scrap iron or steel
Metallized titanium
Thermal sprayed zinc and aluminum
Magnetite
Aluminum
Deep anode installations are those where anodes are installed
at the bottom of a drilled hole.
Deep anodes are at least 15.24 m (50 ft) deep
A horizontal groundbed is installed similar to a vertical installation.
The anode installation details depend upon the spacing of the
anodes. If the spacing is more than 10 feet, it is usually best to
excavate a separate hole for each anode. With closer spacing, it is
usually more economical to excavate one long trench for both the
header cable and the anodes.
Potential measurement test stations are used to monitor the
effectiveness of cathodic protection, check for stray current
effects, and, on unprotected or partially protected pipelines, to
locate areas of active corrosion.
Test station
CIS/DCVG
CIS
CIS
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